(1) Field of the Invention
The invention relates to a process and an installation for anaerobic degradation of waste containing organic solids, in the form of a paste having a high solid concentration, in any case higher than 15%, and in particular ranging between 25% and 30%.
The present invention relates more particularly to the field of the anaerobic degradation of waste formed of heterogeneous organic solids which can contain undesirable particles, in particular heavy and non-organic particles, likely to settle in a fermentation tank, such as for example stones, glass or metal compounds.
(2) Description of the Prior Art
Solid organic waste the degradation of which is the objective of the present invention is previously prepared in the form of a paste with high solid concentration, whereby said paste may be fibrous, but in any case compact.
In the field of the anaerobic degradation of effluents with low solid content, which are hence more liquid than pasty, are known processes for anaerobic degradation using adapted tanks.
FR 2 510 605 for example discloses a process and an installation for degradation in wet anaerobic medium of organic products, by-products and waste, comprising a reactor having a cylindrical fermentation tank vertically divided into two parts by a central partition. The first part is connected by a siphon to a supply well and the second part is connected by a siphon to a discharge well for the material. The supply and the discharge of the products occur through pneumatic thrust.
FR 2 530 659 takes this same structure and proposes to improve it by submitting the effluents to a direction of tormented circulation inside the tank, while providing biogas injection through short and successive jets through conduits ending onto the bottom of the tank.
According to an embodiment of the tank described in these documents, the supply and discharge wells are located in the vicinity of each other, the above mentioned partition being arranged vertically between the two openings of said well ending into the reactor. This partition has a width smaller than the width of the tank and a height smaller than the height of the tank, the bottom of the tank having a double slope having substantially the shape of an ellipse.
According to another embodiment of the tank also described in these documents, the above-mentioned supply and discharge wells are substantially diametrically opposite each other, the vertical partition separating the fermentation tank substantially diametrically, with a height smaller than the height of the tank and leaving communicating passages between the two compartments in the upper and lower portions in order to favour an upward movement of the material in the first portion and a downward movement in the second portion, the bottom of the tank having one single slope.
Even more specifically, FR 2 551 457 proposes to subdivide the enclosure into a plurality of sectors through intermittent injection of biogas, taken from an appropriate storage tank, into each one of said sectors under a predetermined pressure and time period. The biogas is re-injected into each sector successively, i.e. shifted in time, so as to achieve a rotation of the biogas injection into the enclosure, from one sector to the next one.
Finally, FR 2 577 940 proposes to remove the material-supply and discharge wells in order to reduce the construction costs. In this case the products to be degraded are injected directly into the enclosure, preferably towards the bottom of said enclosure, and the exit of the degraded products occurs by gravity. The mechanical thrust is carried out by a pump for thick material, preferably with piston or screw.
One of the disadvantages of the prior known solutions lies in the complexity of the digesters resulting from same. In particular, the manufacture of these digesters is expensive, because of the constraints of internal partitioning and the specific designs of the means for supplying and discharging effluents.
Indeed, when such digesters are used with a paste having a high solid concentration, the partitions carried out must have high mechanical strength because of the pressures exerted by the thick paste in movement. This results into high manufacturing costs.
In fact, the manufacturing constraints on the state-of-the-art digesters increase according to the increase of the size of the tank of the digester.
It also appears from this statement of the state of the art that one of the problems not perfectly solved by the existing processes and digesting devices is the control of a homogeneous circulation of the material to be digested between the supply and discharge paths.
The circulation of the material in the form of effluents occurred so far through partitioning and a tormented control of the flow of effluents inside said tank.